The present invention relates to methods and devices for improving ventricular function of the heart and, more particularly, to an in-vivo method and device for improving diastolic function of the left ventricle of the heart.
Heart failure is commonly defined as the inability of the left ventricle, herein, also referred to as LV, to generate an adequate cardiac output at rest or during exertion, while operating at a normal or enhanced LV filling pressure. Congestive heart failure (CHF) is a clinical syndrome in which heart failure is accompanied by the symptoms and signs of pulmonary and/or peripheral congestion. Heart failure is most commonly associated with impaired LV systolic function. A widely used index for quantifying systolic function is ‘ejection fraction’ (EF), defined as the ratio of stroke volume to end-diastolic volume, which can be estimated using techniques such as radiocontrast, radionuclide angiography, and/or, echocardiography. The normal value of EF is 0.67±0.08, which is frequently depressed in systolic heart failure even when the stroke volume is normal. A value of EF≧0.50 is commonly used as an indicator of normal systolic function. It is notable, however, that as much as 30–50% of all patients with typical symptoms of congestive heart failure have a normal or slightly reduced ejection fraction, that is, a value of EF≧0.45.
In these patients, diastolic dysfunction is implicated as a major contributor of congestive heart failure. In some patients, systolic and diastolic heart failure coexist. The most common form of heart failure, the one caused by coronary arteriosclerosis, is an example of combined systolic and diastolic failure, as described in “Braunwald's Heart Disease: Review and Assessment”, third edition, 1997, Saunders Company Publishers. There are about 4.6 million people in the United States with heart failure, and about 550,000 are being reported annually, as indicated by Vasan, R. S., and Benjamin, E. J., in “Diastolic Heart Failure—No Time To Relax”, New England Journal of Medicine 2001, 344: 56–59. Also indicated therein, is that the mortality rate from diastolic heart failure (DHF), 5–12% annually, is about four times that among persons without heart failure and half that among patients with systolic heart failure, and that, nonetheless, rates of hospitalization and health care associated with diastolic heart failure rival those associated with systolic heart failure.
Primary diastolic dysfunction is typically observed in patients with hypertension and hypertrophic or restrictive cardiomyopathy, but can also occur in a variety of other clinical disorders and has a particularly high prevalence in the elderly population. Aging is associated with ‘physiologic’ diastolic dysfunction due to the increase in LV muscle mass and changes in passive elastic properties of the myocardium, hence, the concern of an increase in the incidence of diastolic dysfunction as the aging of the western world population progresses.
For the purpose of clearly understanding, and implementing, the following described preferred embodiments of the present invention, relevant details, description, and, definitions of selected terms, well known to one of ordinary skill in the art, of physiological and pathological aspects, mechanisms, and functions, of the heart, in general, and of the ventricles and atria, in particular, are provided herein. Additional details, description, and, definitions of terms, thereof, are readily available in the scientific literature.
The left ventricle is the chamber on the left side of the heart that receives oxygenated arterial blood from the left atrium and contracts to drive it into the aorta for distribution to the body. The right ventricle is the chamber on the right side of the heart that receives deoxygenated venous blood from the right atrium and drives it into the pulmonary artery in order to receive oxygen from the lungs. Diastole is the normal rhythmically occurring relaxation and dilatation (stretching, expansion, dilation) of the heart cavities (ventricles), during which the cavities are filled with blood. Atrial contraction occurs during the last stage of diastole of the ventricle and aids ventricular filling. Systole is the rhythmic contraction of the heart, especially of the ventricles, by which blood is driven through the aorta and pulmonary artery after each dilation or diastole.
Ventricular filling starts just after mitral valve opening. As the LV pressure decreases below that in the left atrium, the phase of rapid or early filling of the LV accounts for most of ventricular filling. LV filling temporarily stops as pressure in the atrium and left ventricle equalize, commonly known as the phase of diastasis, occurring prior to atrial contraction and during which little blood enters the filled left ventricle. Atrial contraction increases the pressure gradient from the atrium to the left ventricle to renew filling. When the LV fails to relax normally, as in ‘LV hypertrophy’, increased atrial contraction can enhance late filling. Relaxation (inactivation of contraction) is a dynamic process that begins at the termination of contraction and occurs during isovolumetric relaxation and early ventricular filling. ‘Myocardial elasticity’ is the change in muscle length for a given change in force. ‘Ventricular compliance’ is the change in ventricular volume for a given change in pressure, and, ‘ventricular stiffness’ is the inverse of compliance.
The ‘preload’ is the load present before contraction has started and is provided by the venous return that fills the ventricle during diastole. The ‘Frank Starling law of the heart’ states that the larger the volume of the heart, the greater the energy of its contraction and hence the stroke volume is larger. In other words, when the preload increases, the left ventricle distends (widens, expands) and the stroke volume increases, as described by Opie, H. L., in “The Heart Physiology, From Cell To Circulation”, third edition, Lippincott-Raven publishers, 1998. The pressure-volume relation curves are an accepted description of the ventricular function.
FIG. 1, adapted from the previously cited “Braunwald's Heart Disease: Review and Assessment” reference, is a schematic diagram illustrating a typical pressure-volume loop of a normal subject (dotted line) and a patient with diastolic dysfunction (solid line), wherein dashed lines, between the letters ‘a’ and ‘b’, and, ‘c’ and ‘d’, represent the diastolic pressure-volume relation of the normal subject, and, the patient with diastolic dysfunction, respectively. FIG. 1 shows that isolated diastolic dysfunction is characterized by a shift in the pressure-volume loop to the left. Contractile performance is normal, associated with an ejection fraction (EF) value ≧0.45, with a normal or slightly decreased stroke volume. However, LV (left ventricular) pressures throughout diastole are increased, at a common diastolic volume equal to about 70 ml/m2. In the patient with diastolic failure, LV end diastolic pressure is about 25 mm Hg, compared with an LV end diastolic pressure of about 5 mm Hg in the normal subject. Thus, diastolic dysfunction increases the modulus of chamber stiffness. A main objective of treating the patient with diastolic dysfunction is to cause the diastolic pressure-volume relation curve (dashed line between ‘c’ and ‘d’) to go back to the diastolic pressure-volume relation curve (dashed line between ‘a’ and ‘b’, also indicated by the arrow), of the normal subject, by decreasing the end diastolic LV pressure for the same LV volume.
The fundamental problem in diastolic heart failure (DHF) is the inability of the left ventricle to accommodate blood volume during diastole at low filling pressures, as described by Mandinov, L., Eberli, F. R., Seiler, C., and Hess, M. O., in “Diastolic Heart Failure”, Cardiovacular Res. 2000, 45: 813–825. Initially, hemodynamic changes may be manifested only in an upward displacement of the diastolic pressure-volume curve in the presence of a normal end-diastolic volume with inappropriate elevation of LV diastolic, left atrial and pulmonocapillary pressure (as previously described above, with reference to FIG. 1). More severe resistance to LV filling may cause inadequate filling even in enhanced diastolic pressure with an additional leftward shift of the diastolic pressure-volume relation, resulting in a decreased end diastolic volume and depressed stroke volume, as described by Mandinov, L., et al.
Currently, four different pathophysiological mechanisms are known and used for understanding and/or explaining diastolic heart failure (DHF), combinations of which may readily take place in a particular patient: (1) slow isovolumic left ventricular relaxation, (2) slow early left ventricular filling, (3) reduced left ventricular diastolic distensibility, and, (4) increased left ventricular chamber stiffness or increased myocardial muscle stiffness, as described in the report, “How To Diagnose Diastolic Heart Failure: European Study Group On Diastolic Heart Failure”, European Heart Journal, 1998, 19: 990–1003.
Slow isovolumic left ventricular relaxation, (1), refers to a longer time interval between aortic valve closure and mitral valve opening and a lower negative peak ventricular dP/dt. Regional variation in the onset, rate, and extent of myocardial lengthening is referred to as ‘diastolic asynergy’; temporal dispersion of relaxation, with some fibers commencing to lengthen later than others, is referred to as ‘asynchrony’. Slow early left ventricular filling, (2), is a result of slow myocardial relaxation, segmental incoordination related to coronary artery disease and the atrioventricular pressure gradient. Reduced left ventricular diastolic distensibility, (3), refers to an upward shift of the LV pressure-volume relation on the pressure-volume plot, irrespective of a simultaneous change in slope. Reduction in LV end diastolic distensibility is usually caused by extrinsic compression of the ventricles as in cardiac tamponade. Increased LV chamber stiffness or increased myocardial muscle stiffness, (4), as manifested by a shift to a steeper ventricular pressure-volume curve, is due to processes such as ventricular hypertrophy, endomyocardial fibrosis, disorders with myocardial infiltration (for example, amyloidosis) and replacement of normal, distensible myocardium with non-distensible fibrous scar tissue in healed infarct zones.
The previously cited European Study Group proposed criteria for the diagnosis of DHF. Accordingly, simultaneous presence of the following three criteria is considered obligatory for establishing a diagnosis of DHF: (1) evidence of CHF, (2) normal or mildly abnormal LV systolic function, (3) evidence of abnormal LV relaxation, filling, diastolic distensibility, or, diastolic stiffness.
Pulmonary edema is the result of the increase in pulmocapillary pressure and is due to a shift of liquid from the intravascular compartment to the lung interstitial compartment. Pulmonary edema is frequently associated with hypertension. Gandhi, S. K. et al., in “The Pathogenesis Of Acute Pulmonary Edema Associated With Hypertension”, New England Journal of Medicine, 2001, 344: 17–22, have contradicted the hypothesis that pulmonary edema, apparently associated with hypertension, in patients with preserved ejection fraction, is due to transient systolic dysfunction. They found that the LV ejection fraction and the extent of regional wall motion measured during the acute episode of hypertensive pulmonary edema were similar to those measured after the resolution of the congestion, when the blood pressure was controlled, thus concluding that the pulmonary edema was due to diastolic rather than systolic heart failure.
The management of diastolic heart failure is difficult. There have been no large-scale, randomized controlled trials of therapy in diastolic heart failure, and there remains substantial disagreement about the appropriate therapy for this disease, according to Sweitzer, N. K., and Stevenson, L. W., in “Diastolic heart Failure: Miles To Go Before We Sleep”, American Journal of Medicine, 2000, 109: 683–685. Medical therapy of diastolic dysfunction is often empirical and lacks clear-cut pathophysiologic concepts, as indicated in previously cited Mandinov, L. et al. No single drug presently exists which selectively enhances myocardial relaxation without negative effects on LV contractility or pump function, and thus, there is a significant need for a new therapeutic approach for this particular type of heart disease.
Treatment of diastolic heart failure may be logically divided into three areas or categories: (1) removal of the precipitating cause, (2) correction of the underlying cause, and, (3) control of the congestive heart failure state. Treatment goals that have been advocated, by previously cited Mandinov, L. et al., and, by Braunwald, E., in “Heart Failure”, Harrison's Principles of Internal Medicine, fourteenth edition, McGraw Hill publishers, are as follows:
1. Reduction of central blood volume. Reduction of salt intake and use of diuretics (usually, loop diuretics). Diuretics are effective in reducing pulmonary congestion, shifting the pressure-volume relation downwards. However, they must be used with care because the volume sensitivity of patients with diastolic dysfunction bears the risk that excessive diuresis may result in a sudden drop in stroke volume. Because of the steep pressure-volume relationship, a small decrease in diastolic volume will cause a large decrease of the filling pressure, and will result in a drop in stroke volume, and thus, in cardiac output.
2. Reduction of workload. Reduction of physical activity, maintenance of emotional rest and use of vasodilators. Vasodilators, such as sodium nitroprusside or ACE inhibitors reduce the filling pressure and the afterload in all patients, and elevate cardiac output. Reduction of an elevated left ventricular end diastolic pressure may improve subendocardial perfusion, thus improving myocardial contraction. Nonetheless, vasodilators have not been useful in the management of isolated diastolic heart failure and are more effective in combined heart failure, as indicated in the previously cited Braunwald, E. text. Vigorous control of hypertension is imperative in patients with heart failure caused by diastolic dysfunction, because control of hypertension may prevent progression or partially reverse the disorder by addressing the primary cause of most cases, as described by Grauner, K., in “Heart Failure, Diastolic Dysfunction And The Role Of The Family Physician”, American Family Physician, 2001, 63: 1483–1486.
3. Improvement of LV relaxation. In particular, by using calcium channel blockers or ACE inhibitors. Ca2+ channel blockers have been shown to improve myocardial relaxation and enhance diastolic filling. These drugs may be best matched to the pathophysiology of relaxation disturbances due to their ability to decrease cytoplasmic calcium concentration and reduce afterload. However, currently, use of Ca2+ channel blockers is limited due to their negative inotropic effects (negative influence on the systolic function of the heart), and clinical trials have not clearly proven them to be beneficial.
4. Regression of LV hypertrophy. In particular, decrease in wall thickness and removal of excess collagen by ACE inhibitors and AT-2 antagonists or Spironolactone. Philbin, E. F., Rocco, T. A., Lindenmuth, N. W., Ulrich, K., and Jenkins, O. L., in “Systolic Versus Diastolic Heart Failure In Community Practice: Clinical Features, Outcomes, And The Use Of ACE Inhibitors”, American Journal of Medicine, 2000, 109: 605–613, have shown that the use of ACE inhibitors in patients with ejection fraction equal to or greater than 0.50 was associated with a better NYHA class (New York Heart Association functional and therapeutic classification for stages of heart failure) after discharge from hospitalization, but had no significant effect on mortality or hospital readmission. ACE inhibitors and AT-2 antagonists effect blood pressure, reduce afterload, and effect the myocardium via the local renin-angiotensin system. These effects are important for regression of LV hypertrophy, and improvement of elastic properties of the myocardium.
5. Maintenance of atrial contraction and control of heart rate. In particular, by using beta-blockers and/or antiarrhythmics. Beta-blockers reduce blood pressure and myocardial hypertrophy. The positive effect on diastolic dysfunction is mainly due to slowing of the heart rate and not to a primary improvement in isovolumic relaxation or the diastolic properties of the left ventricle.
6. NO donors. NO (Nitric Oxide) donors have been shown to exert a relaxant effect on the myocardium, which is associated with a decrease in LV end diastolic pressure. In patients with severe LV hypertrophy, an increased susceptibility to NO donors has been documented, which may be beneficial for the prevention of diastolic dysfunction.
7. Heart transplantation. Heart transplantation is a definitive treatment for end stage heart failure.
8. Biventricular pacing. Biventricular pacing improves uncoordinated contraction due to left bundle branch block or other conduction abnormalities with wide ‘QRS complex’ (P-Q-R-S-T waveform) of an electrocardiogram, which are common in patients with CHF. Morris-Thurgood, J. A., Turner, M. S., Nightingale, A. K., Masani, N., Mumford, C., and, Frenneaux, M. P., in “Pacing In Heart Failure: Improved Ventricular Interaction In Diastole Rather Than Systolic Re-synchronization”, Europace 2000, 2: 271–275, have shown that left ventricular pacing acutely benefits congestive heart failure patients with pulmonary capillary wedge pressure greater than 15 mm Hg, irrespective of left bundle branch block. They suggested the beneficial mechanism might be related to an improvement of ventricular interaction in diastole (VID) rather than ventricular systolic re-synchronization. According to their suggestion, LV pacing in patients with high LV end diastolic pressure, will delay right ventricular filling and allow greater LV filling before the onset of VID. Biventricular pacing, however, has not been clinically proven effective in the treatment of patients with diastolic heart failure.
To one of ordinary skill in the art, there is thus a need for, and it would be highly advantageous to have an in-vivo method and device for improving diastolic function of the left ventricle of the heart, while minimally disturbing systolic function of the heart. Moreover, there is a need for such a method and device which is biocompatible and is specially configured for compact and long-term reliable use in humans.